Caption: Bladder stone. Coloured frontal computed tomography (CT) scan of the pelvis of a patient with a bladder stone (small yellow spot, lower right). These stones form when the urine becomes saturated with salts and minerals, such as uric acid and calcium oxalate, which crystallise and grow over time. Stones can form anywhere in the urinary tract, and cause painful urination and blood in the urine (haematuria). Often, no medical intervention is required as the stones may pass out with urine. The stones can be broken apart non-invasively using shock wave lithotripsy, or ureteroscopy, where a thin surgical instrument is inserted into the urinary tract to locate and break up the stones.